Axion assisted Schwinger effect

Abstract

We point out an enhancement of the pair production rate of charged fermions in a strong electric field in the presence of time dependent classical axion-like background field, which we call axion assisted Schwinger effect. While the standard Schwinger production rate is proportional to exp(−π(m 2 +p 2 T )/E ), with m and p T denoting the fermion mass and its momentum transverse to the electric field E , the axion assisted Schwinger effect can be enhanced at large momenta to exp(−πm 2 /E ). The origin of this enhancement is a coupling between the fermion spin and its momentum, induced by the axion velocity. As a non-trivial validation of our result, we show its invariance under field redefinitions associated with a chiral rotation and successfully reproduce the chiral anomaly equation in the presence of helical electric and magnetic fields. We comment on implications of this result for axion cosmology, focussing on axion inflation and axion dark matter detection.

Semiclassical analysis of axion-assisted and axion-driven pair production

“…where we consider the parameter region k ≥ ∂ 0 θ. The dispersion relation with the axion velocity dependence can be seen also in [5,9,10]. The presence of the axion velocity resolves the degeneracy,…”

“…This is why (III.15) has no interference effect. The same formula (III.14) was proposed as an empirical one in [5]. We gave the derivation of this analytical formula.…”

“…We consider a Dirac field ψ which couples to a U (1) gauge field A µ and an axion(-like particle) φ through the following nonlinear terms, S D = d 4 x i ψγ µ (∂ µ − ieA µ )ψ + ψγ µ γ 5 ∂ µ θ 5 ψ − m ψe 2iθmγ 5 ψ , (II. 1) where θ 5 = c 5 φ/f a and θ m = c m φ/f a , f a is the axion decay constant, and c 5 , c m are the dimensionless coupling constants.…”

“…If the axion velocity changes with time, the pair production occurs even though the electric field is absent; i.e., it is driven by the axion acceleration. We call the former one axion-assisted pair production (we can call it axion-assisted Schwinger effect by following [5]), and call the latter one axion-driven pair production. In the subsequent sections, we study these two types of pair production more specifically.…”

“…found a new type of dynamically-assisted Schwinger effects: they studied the pair production of charged fermions in a constant electric field and a constant axion velocity, and found that the Schwinger effect acquires enhancement and interference effects due to the presence of the axion velocity [5]. They named it axion-assisted Schwinger effect, and empirically proposed an analytical formula which may describe the distribution function of produced fermions.…”